The present invention relates to video signal separating apparatus which are used in video tape recorders, etc., to separate a luminance signal and a color signal from a video signal.
Recently, demand for civilian video tape recorders having high image quality has been increasing, and S-VHS and ED beta recorders are widely used throughout the world which provide high Y (luminance) signal resolution and Y signal/C (color) signal separate input and output. Y/C separating devices which separate Y and C signals from a composite signal increase their importance. For separation of Y/C signals, a comb filter is used on the presumption that no signal changes occur between raster lines. If a signal is changing between lines, a band-pass filter is merely used because the signal may be subjected to dot disturbance or color down-shift causing color spread/blot at color contour on display screen. Therefore, in order to separate the Y and C signals from each other accurately, it is required to detect whether the signal is changing between lines.
One example of the above conventional Y/C separating apparatus will now be described with reference to the drawings.
FIG. 1 shows a conventional Y/C separating apparatus. In FIG. 1, reference numeral 12 denotes a band-pass filter for passing a color signal band; 13, a delay circuit which provides a delay of one horizontal period (1H) for an NTSC signal and a delay of 2H for a PAL signal; 14, an arithmetic operation circuit comprising an adder; 15, a selector; 16, a subtractor; 17, a multiplier having a multiplying factor of 1/2; 18, a switching circuit; and 19, a subtractor.
The operation of such a Y/C separating apparatus will now be described with reference to FIG. 1. Assume that the input signal is an NTSC signal and that the delay circuit 13 provides a delay of 1H for the input signal.
The band pass filter (BPF) 12 separates a color signal component from an input video signal 7A and outputs it as a signal 7B, which is then delayed by the delay circuit 13. The signal 7B and a signal 7C, comprising a delayed version of the signal 7B, from the delay circuit 13 are added by the adder 14. The subtractor 16 subtracts the signal 7C from the signal 7B, and the multiplier 17 multiplies the output from the subtractor 16 by a factor of 1/2 and outputs the result as a signal 7F constituting a color output signal from the comb filter. The output signal 7D from the adder 14 is inputted to the selector 15. If the amplitude level of the input signal is smaller than a predetermined value, the selector 15 determines that there is no change or correlation between the signals 7B and 7C and thus the output 7C from the switching circuit 18 becomes the output 7F from the multiplier 17 which comprises a color signal output. If the amplitude level of the signal input to the selector 15 is higher than the predetermined value, the selector determines that there is a change or correlation between the signals 7B and 7C, so that the output 7G from the switching circuit 18 assumes the signal 7B as the color signal. The subtractor 19 subtracts the color signal 7G from the video signal 7A and outputs the result as a luminance signal.
FIGS. 2 and 3 show the waveforms of the input and output signals related to the respective blocks of FIG. 1. FIG. 2 shows the waveforms of the signals in phase with the burst signal and bearing a color component and those which change their states bearing no color components. The waveforms 7a-7g shown below the corresponding waveforms 7A-7G represent the phase differences of starting portions of the respective bursts and signal components relative to the initial 1H of the waveform 7A which is a reference phase. For an NTSC signal, the phase of each burst signal is inverted at intervals of 1H. The signal portions bear a given color, so that the phases of the signal portions are inverted at intervals of 1H. With such signals, the signal 7B is opposite in phase to the signal 7C which has passed the delay circuit 13 during the time interval in which the signals bear a given color, so that the sum of these signals produces a zero level forming a signal 7D. Since the amplitude level of the signal 7D is lower than a predetermined reference level at the selector 15, so that the output signal 7E is caused to select the comb filter output 7F. During time interval t.sub.1 -t.sub.2 in which a signal bearing the given color changes to a signal bearing no color, and when the amplitude level of signal 7D is higher than the predetermined level, the output signal 7E is used to the output from the BPF 12. This avoids color spread/blot at color contour on display screen which would otherwise occur when the output from the comb filter is solely employed.
However, as shown in FIG. 3, if a signal component of a high frequency region of the Y signal is inputted to the apparatus such as causing vertical stripe on the display screen, the signals 7B and 7C are in phase, so that the signal 7D comprising the sum of these signals has twice the amplitude of the signal 7B. Therefore, the amplitude level of the signal 7D exceeds a predetermined reference level, so that the selector 15 determines that there is a change or no correlation between the signals 7B and 7C at the high frequency region signal of the Y signal, and the output signal 7E from the selector serves to select the output from the BPF 12 at the switching circuit 18. Therefore, the high frequency components of the Y signal become a chromatic output 7G which would cause cross color noise undesirably. It is possible to increase the number of circuit elements for determining whether there is a signal change between different lines by increasing a quantity of delay provided by the delay circuit 13 and checking the relation between the different lines, but there would arise a problem of cost increase. There is a similar problem when the separation between Y and C signals is performed digitally.
PAL signal is different from NTSC signal in that the phase of the burst signal becomes inverted at intervals of 2H, so that a quantity of delay provided by the delay circuit 13 is 2H. With the arrangement of FIG. 1, it is possible to avoid such a color deterioration with PAL signal as with NTSC signal, but there is the problem that cross color noise arises due to high frequency components of the Y signal.